As disclosed in, e.g., Japanese Patent Laid-open Publication No. 2005-268763, a plasma processing apparatus using a microwave has a flat dielectric plate made of, e.g., ceramic or quartz and configured to hermetically cover a top opening of a processing chamber. A peripheral portion of the dielectric plate is supported by a support protruding from an inner peripheral surface of a sidewall of the processing chamber. Installed on a top surface of the dielectric plate is, e.g., a metal plate called a slot antenna provided with a multiple number of slots (lengthwise openings) or holes (circular openings). A microwave is supplied to above the dielectric plate, and an electric field is generated below the dielectric plate by the microwave that has leaked through the slots or holes. Then, a processing gas introduced into the processing chamber is excited into plasma by the electric field, and a plasma process is carried out by the plasma.
In such a plasma generation method by the microwave excitation, however, a strong electromagnetic standing wave is generated within the dielectric plate. Especially, high energy plasma tends to be generated at a contact point between the dielectric plate and the support for supporting the dielectric plate, e.g., at a peripheral portion of the support on the inner side of the processing chamber due to a strong electric field. To elaborate the reason for this phenomenon, since a microwave electric field is strong in a region in the vicinity of the dielectric plate, an electron temperature of the plasma increases. If an interface exists in that region, an electric potential difference increases, and, thus, ion irradiation energy to that surface also increases.
As a result, a member near the contact point may be sputtered by the plasma, and sputtered particles may be adhered to the substrate as a processing target object, thereby causing non-uniformity of a processing rate of the processing target object or degradation of a processed material. Further, a plasma property such as radical temperature, plasma density or electron temperature in the vicinity of the contact point may be different from that at the dielectric plate's portions other than the vicinity of the contact point, which results in non-uniform plasma processing.
To solve this problem, in Japanese Patent Laid-open Publication No. 2005-268763, the dielectric plate includes a protruding portion facing the inner peripheral surface of the sidewall with a gap of a preset length provided between the protruding portion and the inner peripheral surface of the sidewall extending from the support. This protruding portion is also called a skirt portion and it is formed as one body with the dielectric plate. That is, the protruding portion is formed by downwardly extending a part of a bottom surface of the dielectric plate. Generation of a strong electric field and high density plasma in the vicinity of the contact point between the dielectric plate and the support for supporting the dielectric plate can be suppressed by providing the gap between the skirt portion and the support.
However, an abnormal electric discharge may be generated between the skirt portion and the support at a certain pressure. Although the skirt portion has a function of suppressing generation of a strong electric field and high-density plasma as mentioned above, an abnormal electric discharge may be generated in the gap between the skirt portion and the support. The reason for this is deemed to be as follows. Since the dielectric plate including the skirt portion is configured as a single body, a portion directly under the dielectric plate and the skirt portion may have the same electric potential. A microwave power is excessively supplied to high-density plasma generated directly under the dielectric plate, and, thus, the abnormal electric discharge is generated between an outer peripheral surface of the skirt portion and the inner peripheral surface of the sidewall of the support, thereby consuming the microwave power. Since an electric discharge may occur in Ar plasma, the abnormal electric discharge needs to be prevented.